text:
What FAA sources of weather information can pilots use to mitigate the risk of flying in icing conditions?
Pilots flying under the FAA jurisdiction have many official sources of weather information available to them. Specifically as it relates to icing conditions, what official sources of weather information can pilots refer to in order to avoid or mitigate the risk of flying in icing conditions?

Here's a list of official FAA weather information sources that pilots can analyze to avoid or mitigate icing risks:
AIRMET Zulu
An AIRMET Zulu will be issued for areas where icing conditions are forecast to be light or moderate.
SIGMET
A SIGMET will be issued for icing conditions forecast to be severe.
PIREPs
PIREPs are the best source of icing information. Unlike weather information sources which are forecasts, PIREPs are reports from pilots flying in the actual conditions and should be considered the most accurate source of icing information.
Here's an example of a PIREP where a pilot of a TBM-700 was descending into Atlanta. He reported a cloud layer topping at 7900 feet down to 500 feet AGL and picking up light rime ice between 7900 and 5000 feet.
UA /OV ATL /TM 1200 /FLUNKN /TP TBM7 /SK OVC015-TOP079 /IC LGT RIME 050-079 /RM DURD

Significant Weather Prognostic Charts
The top two panes of a prog chart help pilots visualize the freezing level. A zig zag line shows when freezing level is at the surface and height contours for the highest freezing level are drawn at 4000 foot intervals with dashed lines. This information gives pilots a forecast of temperatures which when combined with known precipitation can predict ice and help to avoid it.

Area Forecast (FA)
An FA is an additional source of information about where cloud tops and bottoms are. If you know that along with the freezing level you can get a better idea of where icing conditions are most likely.

good answer but you forgot one very important source: the temperature. can't have icing unless you're close to or above the freezing level.

"The temperature" isn't directly part of the answer. The question is about source of weather published by the FAA. The temperature isn't a source of weather. The forecasted freezing level is described in Airmets and is graphically depicted on prog charts. It is also available from FD forecasts, but is most readily available as an indication in the airplane, which isn't directly relevant when discussing weather sources.

in what universe is temperature not part of the weather forecast? its on the TAF, and you can calculate the freezing level using the lapse rate.

If I asked someone, "what sources of weather information are available" and they answered "temperature," I'd be fairly certain that we hadn't successfully communicated with each other. Temperature is a piece of information in the weather sources, but it is not a source of weather information.

You can get it from Airmen's Meteorological Information or Significant Meteorological Information, a compiled graphical version of which can be found at Aviation Weather Center. FAA Advisory Circular 91-74A, Pilot Guide: Flight In Icing Conditions, notes

Aviation meteorologists at the NWS Aviation Weather Center (AWC), local NWS
  Field Offices, major airlines, and private companies prepare icing forecasts. In addition, guidance products are available on the Web. Icing AIRMETs, prepared at AWC, cover a 6-hour forecast window and are updated four times daily.

Also it notes the importance of Pilot Reports (PIREP),

PIREPs remain the major source of information for icing location and severity

Note that FAA requires the weather sources to be approved by US National Weather Service. As far as icing conditions go, to the best of my knowledge FAA doesn't mandate any particular source.


text:
How is ACS different from PTS and when does it replace PTS for FAA certification?
I've read some documents and articles that reference the FAA's new Airman Certification Standards (ACS) that will soon replace Practical Test Standards (PTS). What is different about ACS and when does it take effect?

The Airman Certification Standards (ACS) is basically an 'enhanced' version of the Practical Test Standards (PTS). According to FAA briefing on ACS,

It (ACS) adds task-specific knowledge and risk management elements to each
  PTS Area of Operation and Task.

In general, while the PTS concentrates on what the applicant must 'do', the ACS specifies what he/she must 'know' and 'consider' (in addition to the 'do' part) to qualify for a given airman certificate or rating. The differences (in draft) are shown in this slide.

Image from Update on the ACS by FAA
According to the FAA briefing,

• The “skills” section of the ACS covers the flight proficiency performance
  metrics in today’s PTS.
• For each PTS Area of Operation/Task, the ACS lists the elements of
  aeronautical knowledge that support that skill.
• In addition, for each PTS Area of Operation/Task, the ACS lists the risk
  management elements or behaviors associated with it.

The ACS is still under development, but FAA is targeting a June 2016 implementation (for airplane) depending on results of prototype (there is one running at the Embry-Riddle Aeronautical University) and some other factors.

source: Stack Exchange

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sort: votes


text:
Do I have to take science in high school to become a pilot?
Some articles say we don’t need to take science in high school to become a pilot but some say we do... It’s very confusing and I don’t know which subject to take; Science, commerce or arts.
Hope someone can help.

Which country are you living in?

Just consider whether arts or commerce are going to help you as a pilot. Science is: modern airliners, should you reach that level, are some of the most complex machines ever built, and the way their flight dynamics behave is not intuitive at all. Flying involves a lot of math and physics.

If you wanna fly highly profitably or very beautifully, commerce or arts will be of help. Otherwise, take science.

Welcome to aviation.SE! This is a good question but a lot depends on the country you're in, can you edit your question to add that? And can we assume that you want to be an airline pilot? You might find this question is helpful.

Please clarify what you mean by wanting to become a pilot.  There's a lot of possibilities, ranging from ones where you get paid, such as airline pilots, bush pilots, crop dusters, or various types of military pilots, or just getting a pilot's license and flying your own plane.  For the latter, science is perhaps the better choice, since it gives you a better chance of having sufficient disposable income :-)

My kids had to take at least 3 years of science in high school just to graduate.

There are no specific high school or college academic requirements specified for pilots in the U.S. Code of Federal Regulations.
However, basic knowledge in math and the physical sciences is important, and will greatly improve your understanding of subjects that are taught in ground school so that you can successfully pass required written and oral exams.
Furthermore, to be competitive as a professional pilot a college degree definitely helps, and you will likely need some minimum science credits both to get in, and to graduate.  Don't skimp on your education!

Agree. The earlier you start on science, the easier it will be later on when it gets progressively more complicated...

Do you have to take science? No, possibly not. Should you? Absolutely yes!
Without a basic understanding of science, it will be very difficult to understand the basics of flight as they are all about the balancing of forces (which is physics). Fuel management, air density and atmospherics are easier with a knowledge of physics and chemistry and navigation makes use of physics and mathematics. You could certainly get your private pilots license without any science though navigation will give you problems, but it is very unlikely that you could get a commercial license without a broader education.

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text:
Can I get an EASA class 1 medical if I am red/green color blind?
When I got the Class 2 certificate(one week ago), AME told me that I am red/green color blind and the certificate is valid by day only. Is it possible for me to pursue an aviation career and pass the Class 1 certificate?
I failed almost all red/green Ishihara plates. I heard something about a test called CAD, and if I pass this test I can continue but im not sure.

There may not be a reason that the FAA cannot issue you a first class medical certificate, unless the AME told you on the spot that they could not because of color blindness. It’s possible that An AME still can but with a similar restriction.  This is really more of a question for an AME.

Does this answer your question? In what ways is color blindness an issue in modern jet airliners?

as one of the answers already mentioned reference to «MED.B.075 Colour vision» paragraph of 1178/2011, I won't paste it here again. That is the important piece of legal poetry that makes decision whether you fly at night or not. Or whether you get class 1 medical or not.
Important part is, there is no «color blind» terminology in EASA legislation today. There is «color safe» instead, as it was again mentioned before here in this topic. If you fail Ishihara test (like I did), you either accept your fate by confessing that you are not color safe, or you demand more sophisticated research. I was told by medical practitioner that Ishihara test does not show how severe your red or green deficiency is, it just exposes that such condition exists.
CAD test, being on top of «being very intelligent, super-duper piece of software that is impossible to fool because it is so clever and so many man-years of scientific effort has been put into it»-kind of hype, on the other hand, can show severity of condition somewhat reliably (as they say). There is a certain threshold, degree of deficiency that is acceptable for class 1 medical or class 2 night rating. I failed to resolve 4 Ishihara plates and failed CAD test miserably. Alas, nighttime is sleep time for me :) On the other hand, I have heard of folks that failed Ishihara but passed anomaloscope (see below) and won the class 1 medical prize.
Besides CAD test, you can choose to do a latern test or anomaloscope, which are older tests but still acceptable by authorities AFAIK. I don't remember by heart, but some exotic scrutiny exists in addition to these three. Being successful with any of the above, you get an override of your failed Ishihara test with positive outcome.
All said above applies to EASA jurisdiction.
Some countries have totally different set of rules. US has SODA that can potentially make your life much easier, granted that you actually resolve the colors that are actually needed at nighttime (e.g. you actually see that runway threshold is green and those beacons over there are red etc). Australia is very liberal in this regard and allows color blind people do more in aviation than any other state on this planet.

There are several different tests for color blindness. Ishihara plates are the cheapest but notorious for poor accuracy: if you don’t pass (and many people don’t), that isn’t really a failure; it just means you need a more accurate test.
What other tests are acceptable likely varies by country, but if you can tell red and green traffic lights apart (by color, not by position), then you should be able to pass one of the better tests and get the restriction removed from your medical certificate.

Note: I don't have any links or documentation for this. This is just what an instructor at my flight school said, so take it with a grain of salt.
The only reason a pilot would need color vision is because towers use a red/green/white light gun to communicate with planes that have lost their radio. So, you need to have just enough color vision to be able to tell the difference between the red and green signals. The instructor has to take their color-blind student out to the apron of a towered airport and have the tower shine red and green signals at them, and if they can tell the difference, they're good to go.

I can see red and green properly, the only thing is that I can't pass Ishihara plates test and this is why I have night restriction.

You also need red white color acuity at night in order to accurately judge the orientation of another aircraft in flight.

@CarloFelicione Differentiating between white and colored light is a lot easier than differentiating between two different colors, so I presume the assumption is that, if you can tell red from green, you can tell red from white. But, like I said, I'm just repeating what I've heard, it's possible I'm completely off-base here.

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text:
Is there a regulation that defines when to use QFE or QNH for landing?
Is there an EASA regulation that says to use QFE or QNH when landing?

This question and this one have some related information.

AFAIK there is no EASA regulations forcing a pilot to use one or the other. The default option is to use QNH, which shall be transmitted to all aircraft:

(3) Except  when  it  is  known  that  the  aircraft  has  already  received  the  information  in  a directed transmission, an QNH altimeter setting shall be included in:
(i) the descent clearance, when first cleared to an altitude below the transition level;
(ii) the approach clearance or the clearance to enter the traffic circuit;
(iii) the taxi clearance for departing aircraft.

But there is an option to use QFE as well, and it is commonly used in some places, for example for certain operations in the UK and sometimes for military operations.

(4) A QFE altimeter setting shall be provided to aircraft on request or on a regular basis in accordance with local arrangements.

The "in accordance with local arrangements" part also incidates that EASA has left it up to the individual local authorities to decide whether or not to use QFE - so it is unlikely that EASA has a common rule for this.
Both quotes are from SERA.8015

All UK military use QFE. All civilian aerodromes QNH.

@RobWilkinson that's not true at all. Many civilian aerodromes use QFE (my home base being one of them)

@RobWilkinson that’s not true. Not all Mil aerodromes use QFE.

@Arkhem Perhaps worth a look at UK Mil Regulatory Article 3302(1)

@RobWilkinson perhaps worth a look at the Defence Aerodrome Manual for the largest RAF Station

@Arkhem may be worth checking UK Mil AIP AD2-EGVN-1-10. Details the use of the QNH at Brize Norton under 'Special Procedures' and is highlighted as a Warning as it is untypical of UK Military Operations and is aligned with UK commercial aviation establishments.

So I’m right then? Thanks for clarifying.


text:
What is meant by these criteria for a stabilized approach?
Based on regulation, there are several criteria for when an approach is stabilized. I want to ask:

Only small changes in heading/pitch.

How small?

The aircraft is in correct landing configuration.

What are those configurations?

The aircraft speed is not more than Vref+20 or less Vref.

Is this IAS or CAS or TAS?

As far as I know, stabilized approach criteria are defined by the airline, not in any regulation.

Welcome to aviation.SE! For questions about regulations, please always tell us which country or regulator you're asking about. Regulations can be very different in different places. And when you quote a source, it's always good to tell us what that source is, and link to it if you can.

Although I can’t find regulations regarding stabilized approaches in the FAR/AIM, the unofficial but generally accepted definition of a stabilized approach would be

“Only small changes in heading/pitch.
How small?”
Small changes in heading and pitch would mean changes only necessary to maintain a ground track on the lateral guidance and a consistent airspeed on the glideslope. These are to compensate for drift due to things such as wind.

“The aircraft is in correct landing configuration.
What are those configurations?“
Configuration is entirely dependent on the aircraft and situation.
Typically it would include things like:
Autopilot correctly set or off;
Boost pump on;
Carb heat to hot;
Gas on the fullest tank, both, or on;
Landing gear down and locked with indicator lights lit;
Cowl flaps closed;
Flaps at the correct setting;
Mixture set at best power for density altitude;
Prop speed set at fastest setting;
Landing, position, and anti-collision lights (if not in clouds or fog) on;
All auxiliary and/or unnecessary equipment off;
Sterile cockpit;
Seats and seatback set and locked in the upright position;
Seatbelts and shoulder harnesses on and latched;
Throttle/power set at a consistent setting to provide a descent rate on the glideslope;
Landing checklists complete.

“The aircraft speed is not more than Vref+20 or less Vref.
Is this IAS or CAS or TAS?“
Vref is usually in IAS. Especially since the occasion in which you are using it would be below 3000 feet AGL, and less than 200 knots.

thank you so much for the help! btw for the heading/pitch changes, is there any exact number? ex 3 or 5 degree

Constant rate (vertical speed) descents and constant speed (airspeed) descents are very similar to straight & level, unaccelerated flight. A 1-3° adjustment/compensation is normal. A 3-5° adjustment/compensation is extreme, but not uncommon. Per my instructor, over 5° would be unstabilized. Over 10° would be a deviation from lateral or vertical guidance; lack of directional control; leveling off for a MDA, circling approach or missed approach; or, an actual missed approach or go-around. The rate and frequency at which you make these changes is also a factor. Your instructor may see different.

thank you so much!!!


text:
How can I calculate ambient air pressure from flight data?
I am playing around with data collected during a flight. I would like to find out the (real, not ISA) ambient air pressure for each data point, since it is not reported in the spread sheet containing my flight data.
I have the following data for different time points, but do not know how to (if even possible) derive from this the ambient air pressure:

Mach

Density altitude

Pressure altitude

Airspeed ktas and kcas

Static air temperature

Total air temperature

Geometric altitude

Pressure at the ground / surface pressure

I would be thankful for a solution or any tips for literature helpful to solve this.
Under the following link I found a formula I could rearrange but I believe this would return the pressure in the standard atmosphere...
https://www.weather.gov/media/epz/wxcalc/pressureAltitude.pdf

Not sure what "total air temperature" is.

@RobertDiGiovanni: Some help here.

All you need is the pressure altitude. Pressure altitude is just a convoluted way to express outside pressure. That's how it's computed:
$$\mathrm{Press.Alt.}=\frac{T_0}{L}\left[1-\left(\frac{P_X}{P_0}\right)^\frac{R_sL}{g}\right]\tag{1}$$
where:

$P_X$: Outside temperature in Pa (what you're interested in)
$P_0$: Standard pressure $=101325 \mathrm{~Pa}$
$L$: Temperature lapse $=0.0065 \mathrm{~K/m}$
$T_0$: Standard temperature $=288.15 \mathrm{~K}$
$g$: Gravitational acceleration $\approx 9.81 \mathrm{~m/s}^2$
$R_s$: specific gas constant for dry air $\approx 287.058 \mathrm{~J \cdot kg^{−1}K^{−1}}$

You just need to invert it and get:
$$P_X=P_0\left(1-\frac{L\cdot\mathrm{Press.Alt.}}{T_0}\right)^\frac{g}{R_sL}\tag{2}$$
The result will be in Pa, so remember to convert it to inHg or mbar

I see now the pdf in your question. Yes that's the same equation as equation 1. You already had the answer :)

"real" air pressure

Real air pressure is measured the old fashioned way with mercury in a tube.  Air pressure will lift mercury a certain height in a tube against gravity.  The space above the mercury is a vacuum.  The ratio of the weight of the mercury to the area of the tube cross section is lbs per inch$^2$.
The altimeter is designed to do the same thing, and by setting it to 29.92 one can compare the altimeter reading to standard sea level pressure.  From a graph of pressure vs altitude, "real" pressure in lbs per inch$^2$ can be derived.  The altimeter reading is from static pressure.
So from your list, use "pressure altitude" to compare with "geometric altitude".  Pressure altitude is based on a Kollsman setting of 29.92.  If pressure altitude is higher than geometric, then your ambient pressure will be lower than standard pressure for that geometric altitude.

It kind of depends on how accurate you want to be because you could start asking about instrument errors and position errors and stuff like that.
But, assuming you don't want to get into that...
The link you included would indeed show a standard atmosphere pressure but the pressure altitude data you have is also referenced to the same standard atmosphere (it actually depends on the setting of the altimeter... if it is set to 29.92 / 1013 mBar then you're in luck).
Ori

The list of available data includes “pressure altitude”, which is reading of altimeter set to 29.92 inHg (1013 hPa). As opposed to “baro altitude”, which is the reading of altimeter set to whatever appropriate local altimeter setting.


text:
Must emergency exit door safety pins be removed before flight?
I recently saw while airborne on a flight that the rear right emergency exit door had a pin in it with a red tag stating "REMOVE BEFORE FLIGHT".  The flight was in no rush to depart (had to undergo maintenance for 30min after boarding) so I don't see how it could have been missed.
Is there any reason these should be left in?

Why do you think it would be "missed"? You were boarding the plane when you saw the tag; the plane wasn't flying yet. Part of the departure procedures would be to remove the pin.

No, this was mid-flight.

I suspect OP saw the pin when it is being stored:
https://www.reddit.com/r/aviation/comments/4sg7cw/remove_before_flight_safety_pin_still_in_on/
https://www.youtube.com/watch?v=Jy0X5H7mjc0

@Nelson: That's exactly what I saw. I figured there must be some explanation like that because as you can see from the Reddit picture, you cannot miss it. Do you want to make that an answer?

If you see something like this, this is the pin in storage:

If the pin is in its "deployed" position, it would look like this:

@757toga: Yes. The top picture is exactly what it looked like in flight. And it's right outside the bathroom, so I'm sure flight attendants get asked on every flight "shouldn't this be removed before flight?" Ridiculous design.

Excellent design! By storing it very visibly, you can easily see if it's in the correct position - remember, if it's removed the slide will deploy on opening the door which isn't great on the ground.

Deploying the slide may be what you want on the ground, but it definitely isn't what you want when parked at the gate!

What aircraft type is this? Is it possibly an add on by the airline?


text:
Is a FAA certified plane allowed to fly everywhere in the world?
Following the debate about FAA and Boeing certification practices,  I was wondering whether Europe could potentially require the Boeing 737 Max to be certified by a European agency.
Is a FAA certificate de facto valid in other countries ? Or are all planes required to be certified by local authorities too ?
Update :
Despite FAA certification being valid throughout the world, it appears EASA will conduct its own review of the fixes implemented by Boeing.
It is not a proper re-certification, but the plane will have to comply with EASA in order to be able to fly again in the European airspace.
More about this here (FT paywall).

To answer the assumption behind your question, the 737 Max is quite airworthy. Boeing mucked up the information dissemination part, with tragic results, and did worse by not reacting to the first incidents, but that's on the company, not the plane. The plane would have no problems passing its certifications again, and is quite safe to fly, if only the pilots learn it like a new type rather than just another 737.

In your question, do you mean whether it's legal for a US airline, or person, to fly an FAA-certified plane to France, or whether a French airline can buy a FAA-certified plane and fly it in France? Those are covered under different rules.

@user71659 Whether it's legal for anyone to fly a FAA certified plane in the EU for example.

This may change quickly in the near future. 737 Max incident actually hurts FAA's credibility quite a bit. Those countries that doesn't maintain their own certification agency would probably seek some diversity than blindly endorsing FAA's certification.

@Stanislasdrg You didn't answer the question. Do you mean a) Can American Airlines fly a Boeing to France or b) Can Air France buy a Boeing and fly it in France? The country in which the plane is owned and registered determines the situation.

Not a complete answer, but just keep in mind no-fly zones and active combat zones are generally internationally understood to be barred from anyone but military aircraft. And no-fly zones are only accessible by military aircraft because they're either armed or escorted.

Does "everywhere" include places like North Korea?

It's valid in countries that choose to accept it as de-facto valid but sovereign nations are free to require local certification if they like. The reality is that the FAA has a huge amount of resources at its disposal and for smaller nations is advantageous to simply say we agree that those people know what they are doing and we can't afford to do it any better.
However Boeing does hold an EASA Type Certificate for the 737 as well as the FAA issued one but if you dig into it you will find a lot of "see FAA TCDS" notes. With that said the 737 is certified by European agencies currently. A good chunk of regulations even stem from the FAA work, the best example of this is in the EASA TCDS where EASA changed some regulations to the FAA spec.

However, to harmonize with the FAA, the same requirement was moved to JAR 25.810(a)(1)(ii) at Change 14 which is now in
  CS25.810(a)(1)(ii).

Thank you @Dave. Do you know if the EASA could ask for a re-certification of the Aircraft ?

@Stanislasdrg that may be a good topic for another question but broadly the answer is no: most governing authorities will issue AD's to rectify problems. In some cases aircraft can be grounded until an AD is issued to rectify the problem. They can revoke Airworthiness Certs for non compliant aircraft but if the TC is approved and the aircraft conforms to it, it may be hard to ground it. This is why TC are worth so much in the industry


text:
How does the movement of the cyclic control stick in a helicopter affect the swashplate?
Yesterday I learned that the surface that the helicopter blades create while rotating is very similiar to a gyroscope, and thus gyroscopic precession works there as well. My question is, when the pilot moves the cyclic pitch controls forward, how does that affect the swashplate? Does it rise it in back and lower in front or does it rise on one side and lower on the other?

Horizontal displacement: Tilt the rotor
With the rotor blades rotating around a vertical mast, the total lift vector of the rotor is vertical and the helicopter hovers. If a translation is desired, this vector must be tilted so that an horizontal component can appear. This is done by tilting the rotor disc. The rotor disc is the plane of rotation of the blades.

Rotor tilted forward: The helicopter moves forward.
Rotor tilt: Change blades pitch
The rotor disc is tilted using the lift difference between individual blades. For instance to tilt the rotor forward, each blade creates more lift when it is aft of the mast, less when it is forward of the mast, and the change is progressive between these two positions. The lift difference creates a torque moment on the mast, tilting the mast and the helicopter with it, at least this is our intuition.
As you mentioned, the force doesn't produce its effect instantaneously due to the gyroscopic precession. It is effective only 90° later. So the lift variation must have an advance of phase of 90°. For a forward translation, the lift is maximum when the blade is on the left, and minimum when it is on the right. The momentum is stored into the blade and released 90° later.

The variation of lift is done 90° in advance.
Lift depends on the angle of attack of the blade. Varying the amount of lift according to the position around the mast leads to varying the AoA around the mast: Increased AoA on the left, decreased on the right.

Pitch varies according to the radial position of the blade.
Change blades pitch with the swashplate
Changing the pitch of a blade is done using a system of swashplates. A stationary plate is oriented in space by the pilot using the cyclic stick.
Another plate is maintained in contact with the first one, but can turn freely with the rotor. This second plate is forced to rotate in the plane of the stationary plate, thus it oscillates constantly as it rotates. In turn, it creates an oscillation of the vertical rods (horn links). Each horn converts the rod oscillation into a pitch oscillation on the blade. The blade is now pitched according to its instantaneous radial position relative to the mast, and according to the cyclic stick command.

MBB Bo105, based on photo by Burkhard Domke (source)
Horns are linked to different angular sectors of the swashplate
The vertical rod transmits the stationary swashplate height at its current axial position. By locating the rod 90° ahead of the blade, the link takes care of the rotor precession. The mechanism is so that when the pilot pushes the cyclic forward:

The swashplate is tilted forward.
When a blade moves left of the mast, its horn link measures the height of the stationary swashplate in its aft sector, where the plate is the highest.
The horn is pushed by the link to the maximum, and the blade in turn gets the largest pitch angle.
The situation is reversed when the blade arrives on the right. The link is in contact with the forward sector of the swashplate, where the latter is at the lowest position. The blade gets a low pitch angle.

(Redrawn from source)
Advance for pitch order can be adjusted as required
By adjusting the offset of the horn link which interfaces the horn and the plate, one can adjust to the exact amount of advance required for a given blade.

The swashplate has to be rigged so that the rotor tries to align itself with it. This is because the fuselage is mostly (or completely in case of teetering rotors) just freely hanging under the rotor, so the rotor must not proceed tilting further before the fuselage has caught up with it. When the swashplate tilts forward to fly forward, the rotor tilting forward reduces the angle between it and the swashplate, so the lift asymmetry decreases until the fuselage catches up. If the swashplate tilted right instead, the rotor would continue to tilt and could hit the mechanical limit and break.

Also note that torque transferred to the mast directly by the stiffness of the blade attachments is not subject to the 90° offset, so in many helicopters the offset is not 90°, but only 70°–80°.

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text:
What does the "BA/" stands for in this ATIS?

LTBA ARR
  ATIS V
  0920Z EXP ILS DME APPCH 05 BA/ FOR RWY 05 TRL 110 AFTER VACATING RWY CONTACT 121.8 WIND RWY 05 340 DEG 07KT VRB BTN 310 AND 020 DEG VIS 9999 FEW/02000 FT BKN/03000 FT T 21 OP 15 QNH 1017 NOSIG
  END OF ATIS V

Bracking Action information not available?

I guess so, may be it was just human mistake.

That's what my guess is too. Another (educated) guess: Type: A321, Reg: TC-JRP, ADEP: LTCC, ADES: LTBA, ACID: THY3FN. Correct? :-)

"Sherlock" is that you ? :D


text:
What FAA or other regulations apply to autonomous quadcopters?
I am a software engineer and hobby roboticist and I have just successfully built a quadcopter drone with a full-fledged microcontroller (running firmware that I wrote myself) inside of it, that flies around my backyard, identifies certain things that it finds and sends me reports of what it finds. I do not need to control it with any sort of remote at all.
What legal issues with the FAA and other aviation regulators might I face in trying to bring this to market as a consumer product? Never mind the electronics testing/certification/verification aspect (i.e. all the processes and hoops you have to jump through to have an official "UL" logo on your product). I'm wondering where modern US (likely FAA) drone law stands on completely autonomous, unmanned, non-remote-controlled quadcopter operations for commercial use -- not for delivering the mail like what Amazon is doing; this is just something that flies around on its own in your backyard or in a field, does simple object identification and knows how to steer clear of most obstacles.

Welcome to aviation.SE! If you don't get a good response here you might also try asking on drones.SE.

Well, if the recent NPRM is enacted into regulation, as it likely will be, your system will soon be illegal for sale in the US unless it has a "Remote Identification" system built into it. You need to research Remote ID.  One possible starting point -- https://www.rcgroups.com/model-aircraft-and-drone-advocacy-943/


text:
What do these two METAR codes mean?

METAR YPJT 070300Z AUTO 10010KT 9999 // NCD 27/10 Q1018 RMK
RF00.0/000.0

In the above METAR report, I have two questions:

What does the fraction "27/10" mean?
What does "RF00.0/000.0" also mean?

The term 27/10 is the temperature (27°C) and dew point (10°C).
The RF term in the remark section is used to indicate the accumulated rainfall (in this case 0, so no rain):

What is the meaning of the RF group in METAR/SPECI and TTF?
This group reports rainfall in the last ten minutes and accumulated rainfall since 0900 local. For example, RF02.2/024.4 reports that 2.2mm of rainfall has fallen in the 10 minutes prior to the report time; and 24.4mm has fallen since 0900 local.

(Australian Government - Bureau of Meteorology)


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What are the pros and cons of buying a kit aircraft vs. a factory-built one?
I am a student pilot, and I am trying to decide on what I should do for a plane. I am leaning towards buying a kit, but what are the advantages or disadvantages for that?

Welcome to aviation.SE! I edited your question to (hopefully!) make it clearer; if I got something wrong then don't hesitate to roll back or edit further. I assumed you're asking about which aircraft to buy, rather than which one to train in. FWIW, although this is a good question there are many subjective opinions on it and that means it may not be ideal for this site. You might get a more useful response on a discussion forum where you can hear multiple opinions, clarify your needs in response to follow-up questions etc. The [tour] might help if you're not familiar with how this site works.

IMHO, this is NOT an "opinion-based" question. There are factual, objective advantages and disadvantages to each path here, and they can be presented without descending into the "I like X" and "well I like Y better" debates that Av.SE prefers to avoid. Voting to keep this question open.

Well, you have to build a kit.  The factory one comes with that part done already.  Surely that's in the brochure?  Do you want to build a plane, or do you just want to fly one?  How often?  Do you like building things?  What are you looking for in an aircraft?  The pros and cons are completely subjective (ie: they matter only to you) and we have no idea who you are or what you want out of life.  You have also not shared any of the same with us.

I'm not sure it's true, so I put it as a comment. Here's a story: https://www.youtube.com/watch?v=9zLO5Sem6Ew with details in comments under the video. A man assembled a kit airplane and then found that he became its "manufacturer" which placed certain liability on him for the whole life of the airplane. So it looks like an airplane sold as a kit has long lasting legal consequences.

@sharptooth Do you mean sold as a kit, or bought as a kit, assembled, and then re-sold in an assembled state?  I suspect the latter.

Surely I meant to say that a pilot bought a kit and assembled it himself and maybe tried to sell an assembled airplane later.

The objective is very important.  You don't build an airplane because you want one faster or cheaper.  Neither is the case.  The kit won't be cheaper, and it'll take 1 to 2 thousand manhours, minimum, of your time (take the building time claim of any kit and double or triple it to get reality).
You build one for the experience of building one.  A lot of people who build aircraft go on to build another for that reason (the guy that built the homebuilt I own built two more after mine).
If money's an issue, the most cost effective and efficient way to keep flying after your training is to find one or two partners and buy the nicest creampuff Cessna 150 or similar aircraft you can find (really nice 150s go for around 30k).  With the fixed costs split 2 or 3 ways the expenses don't become painful (if you have to start making financial sacrifices, it takes away the fun - been there done that) and it'll be easy to sell when you want to get something bigger.
If you're mechanically handy and want to maintain an a/c yourself, buy a used homebuilt or LSA.   Only look at a kit because you want to experience the building of the kit.

If you live in the western US, though, a 150/152 is probably going to be a bit underpowered for mid to high elevation fields.  A 172 or Piper Cherokee isn't that much more expensive.

The other problem with 150/152 is the schools buy them up like candy.

So....
Pro: You get to build it yourself!
Con: You have to build it yourself!

In the days of scratch building only about 20 or 30 % of projects started ever got completed.  Kits have increased that but I would say it's still below 50%.

@Jamiec yeah I realize that, which is why I said "and similar".  Non commercial low time 150s do turn up from time to time however.  If the poster wants to be adventurous, I'd advise buying a Classic, like a C-140.  Important to be picky and I always go for the more expensive "creampuff" than the fixer upper if you just want to fly, because the creampuff will cost less and be less hassle in the long run.

@GrimmTheOpiner that sounds like the answer to me!

The big disadvantages of a kit: it will take you years to have an airplane ready to fly, and you'll be flying an airplane built by a novice builder.
The main advantage of a kit: you (often) don't need all the money up front.  Buy the fuselage kit, build it.  Then buy the wing kit, and build that.  Not all kit planes are sold this way, but several are.  No need to pay for the engine, instruments, radios until the airframe is essentially done.
A good compromise is to buy an older, used, factory-built airplane.  You can, with patience, get one of those for the price of a luxury car (50-75k USD, these days), sometimes less than the total price of a kit, instruments, and engine; you'll have something that's more familiar to your mechanic, for which parts are standardized (as opposed to "make it yourself"), with a type certification (meaning you get FAA notifications if a problem crops up that affects an entire type, even if it's 25 years old).
Not to mention, you get to fly something that's, in certain ways, cooler than a brand new airplane, whether factory or kit -- and often easier to fly.  No, a 1948 Cub isn't as fast  as a brand new Velocity -- but it'll be cheaper, cheaper to maintain, cheaper to repair, and cheaper per hour to operate.  And what's cooler than a Cub?

But you can (or could when I last looked) find kits that offer similar performance to the Cub, for instance some of the ones from RANS.

@jamesqf  A quick search found at least 4 companies selling kit versions of the Super Cub.  That's about as similar as you can get.

@Gerry: OTOH, I would hope some of them have taken advantage of 70-odd years of improved materials & engineering :-)

The other answers are good, there's a couple of aspects that haven't been covered that I'll touch on in my answer.
Safety:
Kitplanes have a higher rate of accidents overall than certified airplanes. There's a good article from kitplanes magazine that breaks down causes, it's not authoritative but it's extremely useful in showing the differences in accidents between kits and certified airplanes. One graphic is especially relevant:

Cessna 172s are widely used for training, I suspect the miscontrol rates of certified airplanes are due to that. What really shows clearly is that the major causes of the higher rate are builder mistakes, maintenance mistakes and a higher rate of mechanical problems. If you buy a kit airplane you are getting something amateur built and maintained, certified airplanes are by regulation rigorously tested and built, then thoroughly maintained, all at a cost.
Operating Limitations:
Homebuilt airplanes have more operational limitations imposed on them, what types and what it takes to get around those limitations varies by jurisdiction. In general these limitations are around night and instrument flying, sometimes there's limitations on being able to fly to other or over other countries. In many cases there's extra paperwork that can be filled out to get around these limitations to show required instruments are there and that they work, but it's a consideration you'd want to keep in mind before buying anything.

The homebuilt rules in Canada are actually more liberal than the US in a lot of ways.  Subsequent owners don't need to take the little course to get a "repairman's certificate" to sign off an annual. Whoever's name is on the C of R can sign off anything except a transponder encoder cert.

I'm not a pilot, though I am an aviation enthusiast, and here are some of the pros/cons that have not been mentioned yet.
Kit plane pros:

Kit planes have experimental classification, which means you can add non-certified parts to the aircraft. This can be especially beneficial for things like exterior lights since non-aircraft LED headlights are cheaper and can outperform their aviation counterparts. In general, not being limited to certified parts may allow you to more closely follow modern technology, or maybe even "good enough" technology if that is your thing.
The builder of the plane can do all of the mechanical maintenance on the plane, and do the annual inspections.
You have a lot more freedom to upgrade/modify the aircraft, like adding new engine, prop, etc. to get the performance that you want out of it.

Kit plane cons:

You generally can not use an experimental aircraft for commercial activities. E.g. your friend can not pay you for a ride in your plane (you may split costs, though).

It might be harder to find parts for your aircraft, or mechanics willing to work on it.

You may have a harder time selling it.

One of your "Pros" is also a Con - A "good enough" fix/update/improvement by an amateur may not really be "good enough" and can cost you your life.


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How can I tell if a rental aircraft has been maintained in good condition?
When I rent a Cessna (typically old) from the local airport, how concerned should I be that the plane may have been abused at some points over many years (over-load forces, hard landings, over-rev'd engines, etc.)?
Perhaps the question is "are the plane's standard inspection schedules enough to catch such abuse?". Should I be concerned?

From a fatigue perspective, this isn't really that much of a concern.  The beauty of metal structures is if they get "overloaded" one time, but the overload was not past the permanent deformation point, the yield point, of the affected structure so that nothing is permanently bent, the structure is fine, in general.  A hard landing that did hidden damage to the gear box should get detected at the next annual.
If you do a careful walk around and include gentle pokes and prods with your inspection, looking for loose bits or sloppy clearances, and everything is reasonably tight and straight, the chances of the thing letting go on you for no reason, while you are minding your own business sightseeing, are microscopic.  Look for stories of 172s just falling out of the sky because the wings let go without warning, and you will be looking for a long time.
Cessna once tracked down, back in the 70s, the most beat up high time 172 they could find, and located a 12000 hour airframe that had been used for pipeline patrol, to see how it was holding up.  It just required replacement of hinge bearings IIRC.  Corrosion is the bigger concern than overloads, and I might be wary of a 172 that has spent its entire life tied down outside (it's the daily condensation cycles that's the killer for outside parking unless it's in the desert) unless I was confident that the maintenance at the operation was good.  A 172 that is hangered every night will pretty much go forever.
Same with engines. Lycs and Conts are built like air cooled tractor engines.  Slow turning with large clearances and thick, heavy cranks and connecting rods, dual ignition, simple carburetors.  They usually give off warning signs if they are going to blow up on you (for example, an impending main bearing failure typically results in major amounts of oil coming out the breather line and coating the belly as the excess oil spews out the bearing and fills the crankcase with spray). When Lycomings go bad it's usually their weak point, the camshaft lobes, going south, and this puts metal shavings in the oil filter long before engine operation is affected, which should get caught at annual (as the cam lobe wears down eventually the engine will lose power and misfire).
If it starts nicely, runs nicely making normal static RPM at full power, oil pressure comes up smartly, isn't dumping oil everywhere, no crazy vibrations or scary sounds, the chances of the thing blowing up on you are maybe not microscopic, but pretty small.  Be observant of the FBO's maintenance operation; organization, cleanliness etc.
Learn some of the warning signs of engine trouble (like "morning sickness"; rough running that goes away as the engine warms up, a sure sign of a sticking exhaust valve, and which is guaranteed to get worse).
Statistically speaking, if you are flying around in a halfway decently maintained, straight and tight 172, at 3000 ft on a nice day, with gas and oil in the thing, you really have to work at it to kill yourself.

Thanks, John K!

Regarding the last paragraph: exactly. The most dangerous part of pretty much any aircraft is the pilot. It is extremely rare that the condition of the plane itself if the sole cuplrit to a crash...


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Does paragliding count towards total time?
Does Paragliding count towards flight hours for "Total Time" in either commercial license 250 or the ATP 1500? If not, what are the cheapest ways to acrue raw "total time" hours? Can one do powered parachutes?

What country are you in?

I'm in the united states

It looks like paragliding is part 103 so technically probably wouldn't go under that but I'm unsure on Powered parachutes are they part 61? If so, can you count it towards total flight time?

In the U.S., you can count the time as aeronautical experience in your logbook. But, that aeronautical experience can not be used to satisfy the requirements of Title 14 of the Code of Federal Regulations Part 61 unless the paraglide, powered parachute, or other aircraft meets the following stipulations per Part 61.51:

(j) Aircraft requirements for logging flight time. For a person to log flight time, the time must be acquired in an aircraft that is identified as an aircraft under §61.5(b), and is—
(1) An aircraft of U.S. registry with either a standard or special airworthiness certificate;
(2) An aircraft of foreign registry with an airworthiness certificate that is approved by the aviation authority of a foreign country that is a Member State to the Convention on International Civil Aviation Organization;
(3) A military aircraft under the direct operational control of the U.S. Armed Forces; or
(4) A public aircraft under the direct operational control of a Federal, State, county, or municipal law enforcement agency, if the flight time was acquired by the pilot while engaged on an official law enforcement flight for a Federal, State, County, or Municipal law enforcement agency.

This would eliminate the use of any aircraft flown under Parts 103, 105, & 107.
One less expensive way of getting flight time would be to fly as a safety pilot for someone flying simulated IMC using view limiting devices. If you are instrument rated, you can fly as PIC while someone else is the sole manipulator of the controls of the aircraft in real IMC under IFR.

To supplement Dean's answer, once you have a PPL, the cheapest way to build time is to get a tailwheel checkout, join a glider club and tow gliders. Towing gliders kept me flying in the 90s when I had a young family and probably would have given it up otherwise.
The next best way is to find several partners and buy a good simple airplane like an Aeronca Champ, or an old Cessna 150, ponying up, say, 6000 dollars each or whatever it takes, and fly the hell out of it.  With the fixed costs dispersed among several owners, you will get the total hourly costs well under $50 if you fly it enough.


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Unusual airplane shown in Google Earth

At first I thought it was a plane that had its wings removed for some reason.

Then I thought it might be a lifting body, but I don't think so. Any ideas? This was in Jacksonville, FL; I can provide the coordinates if needed.

Looks like an F-18 with the wings removed. Possibly fire dept training or similar.

Welcome to Av.SE!

Agree with F/A-18, although with a modified radome. Coordinates could help; was this at the Naval Air Station or the international airport? If the former, perhaps trainer of some sort for maintenance technicians, maybe?

@Ralph - not "modified" radome...looks just cut off. Maintenance trainer? No, there's not enough of it left. In all my years of fighter maint (USAF), we just had regular fully functional jets cycle through.

Yeah, it’s the remains of an old F/A-18 classic, with its wings and tailplane removed. It also looks like they removed a substantial amount of the fuselage as well as the engines, rudders and nose radome.  I’m guessing this photo was taken over NAS Jacksonville?


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What is the "tennis ball between cylinders" trick used by flight instructors?
From this answer, "A tennis ball stuffed in between two cylinders -
a common flight instructor trick to see if students are doing a thorough preflight".
A place between the two cylinders is not the expected place for the tennis ball to be, but how does in general this "trick" work? I tried to Google but only pull out the mentioned answer.

If the student does a proper check, they will notice the tennis ball and remove it. If not, well, then the instructor has something to discuss with them.

@J.Hougaard Please post answers as answers, rather than as comments.

From aircraft flying handbook (page 2-9),

The pilot should also check for loose or foreign objects inside the cowling, such as bird nests, shop rags, and/or tools.

This tennis ball is a tool for instructor to see if a pilot does a proper walkaround of the airplane. A pilot should be able to detect this strange object inside the cowling easily if he/she does the walkaround properly.


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Can a light glider without thermal protection land from the orbit, starting from the orbital speed?
I ask about a glider without any special thermal protection (pilot in the space suit), so both answers to the other question do not cover the topic.
When the glider enters the atmosphere, it starts generating lift. Could this lift keep the glider high enough, in low density air, and this way prevent the heat damage? The glider then would fly a long distance, losing speed and altitude slowly, until reaching speed and elevation at which it can safely land. Seems that lots of lift could be generated at speed and air density where the metal burns already.
Is there any analysis ever been done if such landing is possible?

No.  It doesn't matter what the vehicle is, it must enter the atmosphere at high speed, I believe in the order of 2 to 3 km per second!  That is very fast and there is no way to slow it down without generating a lot of heat.  You have to convert kinetic energy into something.

"When the glider enters the atmosphere, it starts generating lift": True but it also compresses air forward to an extreme pressure. Air heats, becomes a magma and the aircraft burns. If you remember radio blackout when a capsule re-enters atmosphere, the reason is the presence of the ionized magma.

The speed is high but the atmosphere is very sparse there. Somewhere very close to the upper boundary, it could potentially generate enough lift to stay high enough to avoid the overheating.

"the atmosphere is very sparse there", hence air doesn't do much to absorb kinetic energy / decrease speed, then air starts to be denser and heats under pressure while energy is transferred. That's why most of the shooting stars burn in atmosphere. Add to this that the range for the entry angle of attack is very small.

@mins: you mean plasma. There is no magma in space.

@TonyK: Yes, thanks! Don't know why magma crossed my mind...

make it 2 gliders, orbiting in opposite directions, connected with a bungee cord. the cord absorbs the kinetic energy, gliders halt and disconnect the cord, then just fall down, baumgartner style ;-)

@szulat: There are two problems: 1/ The cord must be very resistant as it must stop the equivalent of a glider flying at 6 km/s, let say the glider has a mass of 100 kg, this is 3.6 gigajoules of kinetic energy, about the same than a B767 flying at 400 kt. Good luck. 2/ Then you have the acceleration in free fall under gravity, after 5 minutes, the glider will be at 1,3 km/s already. Nothing has really changed.

@Simon Low Earth orbital velocity is around 7 km/s.

For small objects, there is probably be a 'critical aircraft density' below which a return from orbit is possible. A balsa wood model glider would probably land unscathed after a very long re-entry... The same may be expected from paper plane. The small mass will mean that little energy would be dissipated, and the large drag and long re-entry would ensure that its dissipation rate ('power at reentry') is kept low enough...

this is a bunch of hypothesis with no actual data. this should have been a comment.

No.
The stagnation point temperature goes up with the square of true air speed.
Temperature dissipation is proportional to true air speed and density.
Lift is proportional to the square of airspeed and density.
The lower wing loading of a glider (compared to the Shuttle, for example) means that all of the reentry will be slower, but that is not necessarily an advantage. Note that the Shuttle needed external cooling to prevent heat stored in the heat shield to dissipate into the Shuttle's structure. If the glider flies a longer reentry, it will be closer to thermal equilibrium. Also, the aerodynamic shape of a glider is a disadvantage here because the lowest heat load is possible with a blunt object - this is why reentry vehicles look like they do.
Just to give you an idea what temperatures are involved:
If we express speed as Mach number, a typical reentry speed would be Mach 25. This makes the reentry temperature at the stagnation point $$T_s = T_{\infty}\cdot \left(1 + \frac{\gamma-1}{2}\cdot Ma^2 \right) = 126\cdot T_{\infty}$$
at Mach 25. If we assume 195K at the edge of space, this comes out to 24,570K - theoretically, because ionization effects will cause the eventual temperature to be lower and the ratio of specific heats $\gamma = c_p/c_v$ is no longer constant. To calculate the real value needs a non-equilibrium gas model since the nose radius of typical glider wings and tail surfaces is in the order of one centimeter. The distance between the detached shock wave and the structure is too small for the gas molecules to reach equilibrium.
On the other hand, the epoxy resins used in glider construction cure at room temperature first and are then tempered at about 60°C. The glass transition temperature $\text{T}_g$ of such resins is at best a couple of degrees above the curing temperature. $\text{T}_g$ marks the onset of weakening of the composite matrix, and heating the glider above it will permanently damage it. Given that the glider will stay for dozens of minutes in air of a temperature of several thousand degrees and has no thermal protection will make sure that all which reaches the ground is a charred lump of material.

Can you please clarify this sentence that I cannot understand "Note that the Shuttle needed external cooling to prevent heat stored in the heat shield to dissipate into the Shuttle's structure". What external cooling would be available during the descent other that using the rest of the shuttle as a heat sink?

@AdrianHHH: Right after landing the shuttle had to be plugged into an external cooling unit that would feed Freon lines running through the shuttle's avionics bays and hydraulics. The heat source were both the internal systems and the heat shield.

Compare What are the top temperatures occurring during reentry? on [space.se].

Reportedly this is why SpaceX's Starship abandoned a composite hull for a steel one.  Carbon fiber layup is much worse than steel when it's very cold or very hot.

Remember that temperature of a gas is related to the speed of its molecules. When the molecules hit an object moving 25000 feet per second and bounce, that gives them a temperature of many, many thousands of degrees (C or F). A balsa object would char and be destroyed. A glider would have to be designed to have good lift to drag at Mach 22+, not a minor trick.
And when it loses only 30% of its speed, it will need to support half its weight as it will no longer have orbital speed. That equates to significant lift, thus significant drag and since power is speed times force, very large dissipated power that will appear as heating.
So, sorry, but heat shielding is necessary to prevent thermal destruction.


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Why is rotating engines in opposite directions more important for a propeller-driven plane than a jet?
From here it is seen that many multi-engine propeller planes spin their propellers in opposite directions. To avoid producing "left" and "right" types of the engine, a gearbox is fitted.
In contrast, jet engines usually spin in the same direction. Why is the spinning direction less important for a jet engine than for a propeller engine?
A Boeing 777 may have a turbine fan as large as 325 cm in diameter (GE90-115B). Not every propeller is that large.

This is sort of explained in the linked question already. Specifically this answer (which I consider better than the accepted one above it) says:

Today, high power propeller aircraft tend to use identical engines but left- and right-handed gearboxes so the propellers run in both directions. This is less due to gyro effects and mostly to produce benign stall characteristics. The prop wash of a propeller increases the local angle of attack on the wing on one side and decreases it on the other side, so the wing will stall first on the side with bigger angle of attack. If this side is always to the right of the propellers, the aircraft will roll right in a stall.

In a turbofan the nozzle directs the flow below the wing aft of the leading edge, focuses it more and uses guide vanes to stop its rotation, so this effect does not occur. This eliminates the most important factor for deciding to use counter-rotating propellers.
Combine with the fact that most turbofans are not geared, so the whole engine would have to be mirrored, which would complicate maintenance a lot more than just having mirrored gearboxes and propellers does for turboprops.

I think the reason has more to do with the Propeller (P) factor and the rudder corrections required to over come it. This effect does not arise from gyroscopic effects - it is an aerodynamic phenomenon.
Low airspeed and high angle of attack flight causes the blade descending on one side of the propeller disk to see a lower angle of attack than it sees when ascending on the opposite side of the propeller disk. The blade on the side seeing the higher angle of attack generates more thrust creating a torque about a roughly vertical axis. This torque twists the aircraft in yaw and must be resisted by a force created by a rudder deflection if steady flight is to be maintained.
High power propellers will produce a proportionately high torque about the vertical axis at high angles of attack and low airspeed. Thus, high power aircraft will require a significant rudder input to overcome this torque and keep the aircraft aligned with the relative airflow or the runway or to keep it traveling through the air in a straight line and on a constant heading. When operating at high power and low airspeed the required rudder input can be very considerable. A long climb can give a pilot one very tired leg. Also, on applying power by increasing engine rpm the pilot must also make a coordinated rudder input as the amount of rudder input depends on the speed of the propeller. Counter rotating propellers eliminate this P torque and any corrective rudder input requirement for changes in propeller rpm or aircraft angle of attack.
In addition, pitching and yawing an aircraft can change the angle of entry of air into the propeller disk. Once again, this can cause uneven thrust around the propeller disk creating a torque on the aircraft around some axis other than the one the pilot intended. This torque then requires yet another coordinated input from the pilot to maintain the desired flight path. The axes controls are said to be coupled and pilot inputs become complicated and non-obvious increasing pilot work load.
Finally, coupling of two axes in this way can cause an aircraft to continuously oscillate about the pitch and yaw axes and these oscillations can be out of phase causing the aircraft to continually vary pitch and yaw creating an uncomfortable sensation for those on board.
Counter rotating propellers can eliminate all the effects of P factor.
The inlet ducts of jet engines direct the airflow into the engine and the turbine disks and align it with the turbine shaft axis perpendicular to the plane of the blade disk.
As a result, P factor does not come into play here, jets do not have the problems caused by P factor, and jets do not need counter rotation to overcome it.

You are right about the P-factor, but if you have ever stalled a multi-engined aircraft at power with all props turning in the same direction, you would not write the first sentence. Rudder trim would help with the sore leg; nothing helps when the aircraft rolls violently when it stalls.

It is not necessary for a multi engine propeller aircraft to have engines rotating in opposite direction (counter rotating propellers). As a matter of fact, most of the heavy turboprop aircraft such as the Dash 8, ATR, Fokker 50, Saab 340 etc have engines rotating in the same direction. That is they have co-rotating propellers. Yes, it is possible to have engines rotating the same direction, but have gearboxes to make the propellers turn the other direction. But this system is too complex and expensive.
The main issue with co-rotating propeller aircraft is the issue with the propeller P-factor. The P-factor is caused by the downgoing propeller blades experiencing a higher angle of attack than the up going blades. This causes the thrust vector of the propeller to offset. The result is shown in the picture below. The picture depicts a C-130. It has four turboprope engines and when looking from behind all of the engines rotate clockwise. The black arrows on each of the engines represent the thrust vector on them, while the red arrows represent the net thrust on each side of the aircraft. The black arrows are all shifted to the right because that is the side of the down going blades and it causes the thrust vector to offset.

The result of P-factor is that it requires a rudder input every time you add power. Again, looking at the C-130, you can see the net thrust is more to the right on the right side of the aircraft (looking from behind). So, as you add power, to keep the aircraft on the runway center line or on the right flight trajectory right rudder input will be required. The other issue comes when you have an engine failure. Airplanes with co-rotating propellers have what we call a critical engine. By definition, a critical engine is an engine whose failure will result in the highest unwanted yaw. In case of the C-130, the left outboard engine is the critical engine because if it fails, the highly shifted net thrust on the right side of the aircraft will cause the aircraft to yaw towards left. In two engine co-rotating turboprops it is the same story. The left engine is usually the critical engine. As pilots, it is hardly an issue. I flew Dash 8s for sometime in the start of my airline career. And failing the critical engine is always a part of our check rides carried out every six months. With the critical engine failure, the amount of rudder required increases but it is a non-event for a trained pilot. If you look at transport category turboprops you will notice how big the vertical stabilizer is compared to the size of the aircraft. The reason for this is to account for critical engine failures particularly on take offs.

Take note of the very bulky vertical stabilizer of this Dash 8 Q200. (Source: https://www.airliners.net/photo/USA-Army-Dynamic-Avlease/Bombardier-DHC-8-202Q-Dash-8/2081367)
There are many GA multi engine aircraft with counter rotating aircraft. The aircraft I did my multi engine training, the Piper Seneca II had such engines. It had no effects of torque or P-factor as the thrust lines in both the engines are shifted inwards. It had the handling of a jet aircraft. You could fly it without no real input on the rudder. I believe it is a marketing thing in the GA world. One could say it is far more safer in an engine out condition.
In jet powered aircraft, such tendencies do not exist. There is no torque or P-factor. It keeps a straight line if you add or remove thrust. So, it does not really matter how the engines rotate in jet aircraft.

This answer doesn't actually say why turbofans don't have torque or P-factor. It just says states at the very end that they don't, despite still having rotating blades just like a propeller thus basically missing half the answer, arguably the more important half since P-factor and torque in propellers is well known.


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How does pitot tube shape affect indicated airspeed?
Air flow over wings and engine inlets greatly changes due to a number of factors, such as atmospheric conditions, speed, and the design of the surfaces. On some engines you can see mist behind the leading edge of the nacelle in front of the fan, indicative of various pressures resulting from the shape of the edge. In the same way I would guess that the shape of the pitot tube would affect the indicated airspeed- possibly at higher speeds or with a wider edge less air would enter the tube, giving an incorrect reading to the pilot. So how do various shapes affect the indicated airspeed? How about fast aircraft such as the blackbird VS common commercial aircraft VS slower aircraft like a small Cessna?
simulated airflow over standard pitot tube:
1:

I don't understand your question. Wouldn't this just be corrected for by the device that actually displays the airspeed to the pilot? Dreaded car analogy follows. Car speedometers measure how fast the wheel is rotating and translate that into how fast the car is moving. Obviously, the translation on your car is different from, say, the translation on a farm tractor with huge wheels. But that doesn't mean that either speedometer is wrong: you have to look at the whole system and not just the rotation-measuring-thingy/pitot tube.

@DavidRicherby aircraft don't have speedometers that depend on something so easy to redirect as air, and they don't have a range of speeds from 0 to Mach 2

Sure. But the pitot tube is just a sensor. It gives a signal, which is processed to give the indicated airspeed. If you used a pitot tube with a different response, it would have to be processed differently. The indicated airspeed is an output of the system as a whole, not just the pitot tube. You seem to be comparing different aircraft, seeing that they have different shaped pitot tubes and concluding that they must give different IAS.

@DavidRicherby so you're saying shape isn't important to the reading because no matter what it can just be re calibrated?

Perhaps you should edit your question to ask: Does pitot tube shape affect indicated airspeed?

@Pheric I believe that’s more or less the case, yes. Presumably, you have to choose a shape that gives a good response over the velocity range you need, and so on. But the pitot tube is just one component of a system and your question seems to be assuming it’s the whole system.

@DavidRicherby my question is exactly about what you just mentioned-- "... you have to choose a shape that gives a good response over the velocity range you need..." This is what I'm asking about. Is that a bad question? Are there any articles I haven't found about this?

@Pheric OK. So you need to reword your question to ask about that. However, while I voted to close as unclear, the other four close-voters all said your question is too broad. So it may be that they understood what you were asking and felt that, essentially "What shape does a pitot tube need to be to work at different speed ranges?" is too much to answer in a single question.


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Why isn't the arresting hook always used on land?
Watching Jet Stream, about training to fly the CF-18 Hornet, they comment that the jet was originally certified as a Navy plane, landing on aircraft carriers with an arresting cable.
Usually, when landing on land, they land hard and fast and take a mile of runway to come to a stop; but during an emergency, they use the tailhook which brings the jet to a stop after only 1,000ft.
My question is: why don't they always use this to land?
Here's a short clip I've taken from the show:

There is a lot of stress placed on the airframe when you use the tailhoook. You can see the skin buckling in the picture below behind the landing gear. I'll admit, you probably wouldn't land intentionally with this sort of technique on land but if you catch the wire during the flare it could probably pull you down pretty hard. For these types of landing and it would be difficult to get it right every time.
(Like  I wrote in the comments, I do realize that the aircraft below has not yet caught the cable. When you flare, and you might catch the cable mid-air if you have a nose-high attitude, and slam down hard as it pulls you backwards and you loose lift, or you do a non-flare like carrier pilots, also very hard. IMO I think either would end up like the landing below.)

You have no ability to control the aircraft and you end up stopping and rolling backwards on the runway. You can only be one aircraft at a time and you cannot roll off straight away, wasting time.

You'd have to have an arresting gear system and along with that more staff and maintenance.

At least on aircraft carriers, I believe the arresting cable has a limit of only 250 catches for safety reasons. Even if it the limit was greater on land, you'd have to have people changing the cables on a regular basis.

You'd still have to have conventional brakes which you could just as well use.

The stress ripples in that picture have nothing to do with the tailhook. Have a close look at the cable - it's just been picked up by the hook and hasn't moved 50cm yet. It won't take any significant landing stress until the plane has pulled several meters of cable out of the blocks. More likely it's the weight of the empennage + radome on top bending the fuselage against the main gear via the wing. Same thing would happen if you slam the plane down on land. (Note: not disagreeing that carrier landings are very hard on the aircraft.)

Well, the arresting cable has a strict limit because a broken cable means a lost aircraft. If the cable breaks the aircraft already decelerated below flying speed and now has nothing to stop it, so it will fall overboard and splash down. Pilot(s) will eject (anything that ever lands on carrier has ejection seats, including the pictured E-2), aircraft will be lost.

@paul I added the picture since I see two options- you flare, and you might catch the cable mid-air, and slam down hard as it pulls you backwards and you loose lift, or you do a non-flare like carrier pilots, also very hard. IMO I think either would end up like the landing above.

Modern carriers have 3 or 4 cables that can catch a plane; pilots normally aim for the last available cable. The US Navy only uses the arresting cables for 125 landings. The purchase cables (in the picture, near the block, you can see where the attesting cable quick-connects to the purchase cable) are used considerably longer (attesting cables are hemp-cored for a "softer" landing; purchase cables are straight steel). Most planes automatically go to 100% throttle upon hooking a cable to more easily recover from a mis-hook or other problem.

@Chris S:  Some misinformation here.  Pilots always aim for the middle of the cables which, if done accurately, means that they catch the 3-wire in a four wire system.  Also, the planes go to 100% power on hitting the deck, they do not wait to detect a wire catch prior to throttling up.  This is a routine procedure, but it is not automatic - the pilot does it.

Despite all this, I think the arresting hook could be a viable layer of redundancy on short/slick runways for commercial airliners. Ofc, it would require a lot of modifications to the system.

@shortstheory very difficult and heavy for a situation that virtually never exists (complete brake failure). IMO it's better to use EMAS and/or a net at the end to catch the plane should the need arise.

@shortstheory - No. It's just not practical for an airliner. First, even for a relatively small B737, you're stopping about five times as much mass as an F/A-18. Second, the pilot of an F/A-18 is belted in with a 6-point harness, while airline passengers get a lap belt. Third, the deceleration forces of landing on a carrier are up to 4g, which if applied to the average airline passenger would snap necks, break noses and backs and knock people unconscious. Catching a cable is a controlled crash in many ways; airline pilots have the luxury (in most situations) of more finesse.

There are several reasons why arresting cables aren't used on land:
Stress
As Manfred noted, the stress that gets put onto a plane during landing is incredible.  According to HowStuffWorks "How Aircraft Carriers Work", its not unusual for an aircraft to hit the deck at around 150 mph or about 67 mps.  Since they have less than 500 feet of runway (~150 meters), they must decelerate at an incredible speed to a standstill in about 2 seconds.  That equates to about 3.5 G's!  In addition, Navy pilot's are taught to push the throttle to max upon hitting the deck so that if they miss one of the 4 cables, they can still have enough speed to take off and try again.  This may now happen automatically as Chris S noted, I'm not certain.  This means that the aircraft has to undergo even more stress.  Navy planes must hit the deck so hard that its often compared to a controlled crash instead of landing and aircraft frames must be structurally modified to endure that type of stress.
Difficulty
According to Tom Clancy's Carrier: A Guided Tour of an Aircraft Carrier, it is extremely difficult to catch the wires.  It is so difficult that Navy Pilots are often ranked on how often they hit the 2nd wire (considered to be the best).
Price
As Manfred noted above, the machinery to slow an aircraft is not trivial.  While I'm not familiar of the routine replacement rate for these cables, I would imagine that it would be something else that would have to be monitored.  The cable is actually attached to a large machine which provides the cable with some give.  I can't imagine that they would be cheap, but I'm not familiar with the exact machinery myself.

Regarding price, I'm not certain on the cost of the cross-deck pendants (the "wire" the hook grabs) but as I recall they are life-limited at somewhere around 100 traps. The arresting mechanism (usually hydraulic) also requires maintenance (and in the case of land-based systems, needs to be set up as either a permanent or temporary installation on the runway being used).

Typo: 150 mps or 67 mps?

@NateEldredge 150 Miles Per Hour or 67 Meters Per Second, fixed above.

They are actually going for the 3 wire. Since there are 4 wires, if they can put the hook on deck right in the middle, they will catch the 3 wire. Catching 1 or 2 means you were down early, which means you were a little lower/ steeper in glide slope. Its very important they not come down TOO early, else risk hitting the bomb ramp, which would change everyone's day. If you have to be wrong, be long.

Arrested runway landings are not very difficult and they are not executed in the same fashion as carrier landings.  Runway arrested landings usually only use 1 arrestor pendant and the jet touches down well before it crosses over it and the hook engages it.  Carrier landings on the other hand demand a precise approach to the deck due to the limited space available to arrest the jet while preventing a ramp strike from a low approach and early early touch down.

Fatigue basically.
A carrier landing is often referred to as a (hopefully) controlled crash. Navy aircraft are built much stronger than land-based aircraft for that reason. If you can avoid subjecting your aircraft to those sort of conditions, then why wouldn't you?

Are you suggesting that an exactly similar model of an aircraft would be built separately based on its operations (land or naval)? Isn't the same F-18 supposed to operate anywhere (land or carrier)? How would it be built separately?

@RaajTram The F-18 was buit as a carrier aircraft, so it is built to withstand that sort of use case. One of the reasons that some air forces have chosen the F-18 is exactly because it is over-built for land-based operations. The F-111 had a carrier variant (F-111B) that was considerably heavier (and thus a complete failiure) than the land-based version for this exact reason.

An additional answer not discussed yet is the effect it would have on the runway.
If you've ever seen the deck of the carrier in the impact zone, you know that it is hammered all to hell from the repeated beating by the tail-hooks coming in much steeper than a runway approach (insert controlled crash reference here). For the ship it is less of an issue, because the deck will get regular maintenance and the ship doesn't always need to be ready to fly (carriers actually spend a lot of time getting repairs compared to a runway).
If you were to start hammering away at the concrete on a runway all the time like that steel deck, it's going to tear it up pretty quick, and then you have to deal with not just the dings in the runway, but the pieces that came out of those holes AND with doing patch work on those holes.

Some military runways (on land) do have arresting cables. These are not intended for routine use, but to catch aircraft in danger of running off the end of the runway due to brake failure, touching down too late, etc. Sometimes they may pop-up to snare the undercarriage rather than using a hook, Navy style.
The reasons for not using arresting cables on land have been well discussed above -- airframe and pilot stress from the high G's, maintenance costs, etc. Also, there's usually plenty of land on which to build a runway, so you don't need to stop short as on a carrier.

The military runway I mentioned in the question, as well as the video do have arresting cables for emergency use - hence the question.

@DannyBeckett This is purely speculation, but the runway you showed might be a training runway that the Navy would use for practice.  Do you know where this runway is or what is it used for?

@HardcoreBro It's Cold Lake in Canada. They do use it for training, but they don't normally use the arresting hook. It was used in that video due to an engine failure.

I will add my own answer shortly, but when I served USAF, we had runway cables for use in case the aircraft was experiencing hydraulic failures - which would mean the brakes wouldn't work. Even an F-15, a land-based fighter jet, has a tail hook.

I served USAF as a tactical aircraft (fighter jet) maintenance technician (aka Crew Chief). F-15's are a land-based aircraft, and have an arresting hook, and the runways have arresting cables. However unlike a carrier, there is only a single arresting cable, and it is for emergency use only.
In the event that a fighter jet is experiencing a hydraulic failure that may mean loss of brakes, the arresting hook will be deployed, and the pilot will land short to ensure that it will catch the cable. The cable is SLIGHTLY elevated above the runway, so the hook can properly catch it, but not so much that the wheels won't easily roll right over it.
The arresting hooks on these aircraft are not designed for frequent use. While they are attached more or less directly to the keel of the aircraft to minimize strain on the airframe, the arresting hook has a large solid aluminum pad that must be replaced every time it is used.
The cable on the runway is a drag system, NOT an active system. As I recall from my training it uses inertia reels to aid in slowing down the airplane, but there is enough cable length to allow the airplane to go a decent distance down the runway still. Compare to a aircraft carrier, which has an active restraint system and the cables stop the aircraft almost immediately.
I believe that Navy fields used to train pilots for carrier landings, actually have the full sequence of cables that a carrier has, not just a single cable for emergency use.
There is a famous incident involving an Israeli F-15 that lost a wing during a combat training accident, and was subsequently flown back using afterburner (The pilot didn't know he had lost a wing). Upon landing, he was still going at near-MIL speed and required the arresting cable and arresting hook system to stop the aircraft. Not usually mentioned in the documentaries, the arresting cable system on the runway was severely damaged by the incident.

"I believe that Navy fields used to train pilots for carrier landings, actually have the full sequence of cables that a carrier has, not just a single cable for emergency use."  Not true BTW.  Not necessary because field carrier landing practice consists of touch and goes.  There's no need to practice actually catching a wire, and it would greatly reduce training efficiency.

Many air forces throughout the world in fact do use runway arresting gear during flight operations and a glance through an Airport/Facility Directory will reveal that even many large civilian airports are equipped with runway arresting gear.  It is generally reserved for emergency operations, as seen in the JetStream series, where either the aircraft may have a malfunction which prevents conventional means of braking during landing or situations where portions of an active runway may be damaged, denying flight operation down their full length.  For normal flight operations on long runways the arrested landing is not used, primarily as it more stressful on the airframes, resulting in more maintenance and higher operating costs.
Quick description of runway arresting gear and their use here.

Here's something really wild as well;  Australian F-111 makes a wheels up landing using arresting gear after the landing gear failed to deploy.


text:
What does ''Reduced Coordination Airspace'' mean?
What does ''Reduced Coordination Airspace'' mean?
There is a definition in here, page 4 bottom image.

What exactly is your question? Your title is in question form. Yet, you answer your own question in the rest of your post without an explanation of your confusion or what/why you are asking.

I didn't get the definition exactly; I think it is not detailed so much,  My question is clear what is the Reduced Coordination Airspace' with an one example may be. @Dean F.

Please quote the specific material you want help with, and name the source, in case the link breaks.

In many parts of the world, the military has reserved large parts of airspace for military activities. As a rule of thumb, civilian traffic is not permitted in these areas. Traditionally, such military areas were reserved 24/7, regardless of whethere there actually was any military activity at a given time. This is not very efficient, as it forces civilian flights to make, sometimes long, detours to avoid military airspace, even when the military isn't actively using those areas.
In Europe, the concept of "Flexible Use of Airspace" has been a way to try to reduce these unnecessary detours, in an effort to shorten flight tracks, reduce flight times and fuel burnt for civilian flights. One way to do this is to allow ATC to coordinate civilian flights through military areas (temporary segregated areas) when possible.
Reduced Coordination Airspace takes this one step further, essentially allowing civilian flights to operate within military airspace when there is little or no military activity, without requiring ATC to coordinate individual flights with the military. It basically allows ATC to give shortcuts to civilian flights, taking them through military areas, without having to ask the military for permission for every single flight, thus contributing to a more flexible use of airspace.
The actual definition, from the document you included in the question, is as follows:

Specified portion of airspace implemented when Operational AirTraffic (OAT) is light or has ceased and within which General AirTraffic (GAT) is permitted to operate outside the ATS route structurewithout requiring General Air Traffic (GAT) controllers to initiate coordination with Operational Air Traffic (OAT) controllers.


text:
Why there is a helicopter sitting north of madras, OR, , above the warm springs reservation every single night?
Every night I see a helicopter light above the reservation sitting in a spot above my house, but way up that I can't see it closely, just the light.  Has to be a helicopter since it sits in one spot.

If it stays in one place every single night it's probably a tower of some kind. Towers have anti-collision lights so that pilots can see them at night.

PS if you meant "every single night" please edit as it appears you may be referencing some sort tribal ceremony or something.

Also you might consider editing the question to address some of the questions in my answer.

@quietflyer Typo in title edited (you could have easily done so yourself BTW). If you have clarifying questions for the OP, please add them as comments to the question, not as an answer.

@ J.Hougaard I didn't want to rule out the possibility that the title really did refer to a periodically-occurring, maybe weekly, tribal ceremony.

If you are looking up at a bright thing in the southwestern or western sky before 11 PM in March 2020 and you aren't also able to locate the planet Venus at the same time, then the bright thing is the planet Venus.
Venus is very high and bright in southwestern or western sky in the evenings these days.  Unless it is very near the horizon, it appears to shine with a white steady light.  With a clear distant horizon, it will set in the west about 23:30 PDT at your location.  (Calculated via this on-line tool.)
The star Sirius is also still prominent in the evening sky.  It can appear to flicker quite dramatically, flashing from red to blue, especially when near the horizon, but is nowhere near as bright as Venus.  With a clear distant horizon it will set in the southwest at about 2:50 AM PDT at your location.  (Calculated via this on-line tool.)
Does the object make a helicopter sound, or some other sound, or no sound?
Is the light steady, or is it definitely blinking completely off and then back on?  What color(s)?   What direction is the light from you?  Does it stay in the same direction from you if you drive 10 miles down a road and look at again? (I.e. does it appear to "follow" you as some observers have often said about such things?)  Is it so high that it could not be a tower?  Does it get nearer to the western horizon over the course of several hours? Can you establish what time it "sets" below the horizon, if it does? Is it still visible at midnight? Is it still visible in the pre-dawn twilight? If you have access to binoculars, does it look like a tiny round circle that is not flashing or flickering when seen through the binoculars?
Odds seem good that it is Venus, but the answers to these questions should confirm or reject this hypothesis.  Due to the path of the ecliptic, Venus may be well north of due west even near the spring equinox when the sun is setting nearly due west.
If it is definitely a flying vehicle, which seems unlikely, you can't assume it is not an unmanned drone/ UAV.

Future edit- opening paragraph should reference PDT specifically; also could double-check that Venus setting time is after 11 PDT throughout entire month of March 2020 and if not adjust stated time accordingly.

in the words of Yoda, "edit or do not..."  The rest of us derive no benefit from your notes to self.

@MichaelHall -- I guess I'll wait till the moderators tell me that, or until they start deleting my notes to myself.  The reason they are there is to facilitate collecting multiple edits into one single batch.  I've been specifically asked (warned really) to edit less, and a potential edit that might sit waiting for weeks or months tends to get lost if it is just filed away as a wordpad document or something, perhaps on some different device than the one I will be using at some future time.  I take note of your comment all the same.


text:
When a Tower and the Approach Control closes who takes over the airspace?
When a Tower and the Approach Control closes, who takes over the airspace?
and also what about When a Tower, within an Approach Control, closes, who takes over the airspace?

All airspace is ultimately owned by some Center (US: ARTCC, ICAO: ACC). Some is delegated to Approach facilities (US: TRACON), and Approach can sub-delegate part of their airspace to a Tower. And Center can delegate airspace directly to Towers without an overlying Approach.
If any facility closes, it’s associated airspace automatically reverts back to whoever had delegated it.

All airspace?  There's a lot of uncontrolled airspace in places like the western US, and many places where you couldn't contact the center if you wanted to.

@jamesqf Someone has to own it...

@jamesqf With very few exceptions (https://aviation.stackexchange.com/questions/12810/why-is-there-a-strip-of-uncontrolled-airspace-between-russia-and-norway), all airspace is contained in some FIR, and within a FIR, at least alerting service is provided, regardless of airspace class.

@expeditedescent: The service may be provided, but it's only useful if you're at a high enough altitude to have radio reception.

@jamesqf Sure. But that's not what the question is about

I work at an airport which closes at night. We have a control zone (class D), a TMA (also class D) and some surrounding airspace (class G/E). Tower and Approach control is provided locally during opening hours. When the airport closes, we call the area control centre responsible for the surrounding airspace, and they take over responsibility. In addition, the control zone and TMA actually seize to exist when local ATC is closed, so they both become class G airspace.
I also work at another airport where tower and approach control is provided locally, but two smaller airport have their own AFIS ("tower") below the TMA. When they close, they then call us and hand over the airspace to Approach. One of them becomes self-service when the AFIS is closed, so while Approach is technically responsible for the airspace, we will still advise traffic to switch to the local frequency  to communicate directly with any other pilots in the area.

To add to StevenS answer, a Class D airport will usually revert to a Class E or G airspace after the tower closes. The hours and the subsequent airspace change will be noted in the US Chart Supplement. Most of the Class B airports that I know of generally do not close. Some busy Class C and D airports also do not close. Although, the FAA has reduced hours on many facilities due to reduced traffic.

Lots of “24x7” facilities have closed in the last few months due to COVID cleaning, even Centers.


text:
Does the Antonov An-225 have an Auxiliary Power Unit (APU)?

Most APUs are installed in the rear, see the above image of an Airbus A380 as an example. On the An-225, no APU exhaust is visible at the rear of the aircraft.
So does the An-225 have an APU, and if yes, where is it located?

No. It doesn't have an APU — it has two of them.
They are located close to the main landing gears. From An-225 Mriya is the world’s largest aircraft (English version):

Auxiliary power plant consisting of two TA-12 turbofans installed in the left and right chassis fairings provides independent power to all systems and starts the engines.

I think "turbofans" is not the proper term here. Actually the TA-12 is (just) a gas turbine (Wiki (Russian), Google translation).
The location of the APUs being much closer to the tarmac might be a reason why Copenhagen airport has (or had) special regulations for its operation on an An-225 (source):

Special regulations for AN124 and AN225.
  Aeroplane type AN124 and AN225 must not operate their APU unless the pavement
  underneath is protected against heat and blast effects from the exhaust.

I couldn't find a better picture but the exhaust pipe is quite visible here (red circle):

(Source)
I'm not perfectly sure whether the green circle marks the intake, but I guess so.

The pictures are tremendously beautiful. Thank you for the information and all the pictures.

@AirCraftLover You mean the pictures from the first link? Yes, indeed, they are awesome. To me, the An-225 is one of the most beautiful aircraft ever. Together with the Queen (B747) and the Concorde.

Yes, the first link, containing various picture of the An-225.

actually the picture above is not quite correct, i have seen a video of the AN225 on the taxiway and saw the door near the rear for the APU intake (in blue) and RED for the APU exhaust in this picture.


text:
How is a Class E airport distinguished from a Class G airport, based on the identifier?
Some small uncontrolled airports have "K" identifiers such as KPHK (Palm Beach County Glades) "Pahokee". Others such as 42M (Thayer Mem) "Thayer" have a numeric identifier. Both have 122.9 CTAF listed. Pahokee is Class G, Thayer is E.
My question is how is an "E" airport distinguished from a "G" airport on the basis of the identifier?

Are you referring to the US?

Related

Airport identifiers do not distinguish the airspace that surrounds them. As for why airports start with a K you can look at the answer here. To figure out the airspace that an airport sits in you must look at the airspace designation lines around it (see page 10 here).

Your link to "history" appears to have been changed. It now has just a list of airport codes. I was looking for an explanation of why some US airports have three letters and numbers, and no K at all.

The AFD / Chart Supplement gives further information, especially airports that have different airspace classes at different times.  For example, KMCE has that dashed magenta circle around it on the sectional, and in the chart supplement says "AIRSPACE: CLASS E svc 1600–0100Z‡; other times CLASS G."

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

I disagree.  Perhaps not the greatest or most complete answer ever to appear on this website, but much, much better than some.  And your comment regarding reputation just comes across as snippy.  Let the community decide this.

@MichaelHall It might be worth noting that the above comment with Pondlife's name on it was automatically generated; Pondlife did not write that.

@Pondlife is correct, this is not an answer, but it's good information and entirely appropriate as a comment.  And a real human actually wrote this.

@Tanner Swett, how is a person to know which comments to ascribe to the individual named, and which ones are to be disregarded as the product of runaway AI?  And PJNoes, I won't debate opinion with you.  I didn't say it was a good answer.

Just wondering how many of the people (or bots) who don't like my comment as an answer actually read the AF/D / Chart Supplement.  It literally tells you, for every airport, what class of airspace it's in, and whether it changes when the tower is closed.
Aymhan's original answer stated that the airport code, whether IATA, FAA 3-letter, or alphanumeric does not indicate class of airspace, so that part of the answer was already provided.  The way you tell is by airspace boundary markings on the sectional (answer also already provided), and/or by looking up the airport in the AFD / Chart supp.